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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Elucidating electronic transitions from σ orbitals of liquid n- and branched alkanes by far-ultraviolet spectroscopy and quantum chemical calculations
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Elucidating electronic transitions from σ orbitals of liquid n- and branched alkanes by far-ultraviolet spectroscopy and quantum chemical calculations

机译：通过远紫外光谱和量子化学计算阐明液态正构烷烃和支链烷烃的σ轨道的电子跃迁

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摘要

Attenuated total reflection far-ultraviolet (ATR-FUV) spectra containing Rydberg states of n-alkanes (CmH2m+2; m varies in the range 5-9) and branched alkanes observed in the liquid phase were investigated by quantum chemical calculations with the aim of elucidating electronic transitions from σ orbitals of liquid n- and branched alkanes. New assignments are proposed based on the time-dependent density functional theory (TD-DFT) and symmetry-adapted cluster configuration interaction (SAC-CI) calculations, and the differences in these spectra are analyzed in detail. The FUV spectra of n-alkanes show a broad asymmetric feature near 8.3 eV. The strong band at ～8.3 eV shows a red shift with a significant increase in intensity as the carbon chain length increases, which is attributed to the overlapping transitions from the third (or fourth) highest occupied molecular orbitals HOMO-2 (or HOMO-3) and HOMO-1 to Rydberg 3py by the TD-DFT and SAC-CI calculations. This band was previously assigned to the overlap of two peaks arising from the transition from the HOMO to 3p and from the HOMO-1 to 3s based on their term values. Although the most intense transition, T1, is from HOMO-2 for m = 5 and 6 and HOMO-3 for m varying in the range of 7-9, the shape of Kohn-Sham molecular orbital for T1 is similar among the all-alkanes investigated. The theoretical result also has demonstrated that the red shift originates in both stabilization of the Rydberg 3py and destabilization of the occupied orbitals. The intensity of the shoulder at 7.7 eV drastically increases in the spectra of the branched alkanes, especially for those with quaternary carbon atoms such as 2,2-dimethyl butane. This increase in intensity is caused by a reduction in symmetry in the branched alkanes, which leads the forbidden transitions to Rydberg 3s to allowed transitions. In this way, the present study has provided new insight into the existence of their Rydberg transitions and the shape of the relevant MOs of the transitions.
机译：包含正构烷烃（CmH2m + 2; m在5-9范围内）的Rydberg态的衰减全反射远紫外（ATR-FUV）光谱，旨在通过量子化学计算研究液相中观察到的支链烷烃阐明液态正和支链烷烃的σ轨道的电子跃迁。基于时变密度泛函理论（TD-DFT）和对称适应的簇配置相互作用（SAC-CI）计算，提出了新的任务，并详细分析了这些光谱之间的差异。正构烷烃的FUV光谱在8.3 eV附近显示出广泛的不对称特征。 〜8.3 eV处的强带显示出红移，强度随着碳链长度的增加而显着增加，这归因于第三（或第四）最高占据分子轨道HOMO-2（或HOMO-3）的重叠跃迁），并通过TD-DFT和SAC-CI计算将HOMO-1转换为Rydberg 3py。该波段先前根据其项值被指定为两个峰的重叠，这两个峰是从HOMO到3p的跃迁以及从HOMO-1到3s的跃迁引起的。尽管最强烈的跃迁T1来自于m = 5和6的HOMO-2，而来自m的HOMO-3在7-9范围内变化，但在所有情况下，T1的Kohn-Sham分子轨道的形状相似-烷烃的研究。理论结果还表明，红移既起源于Rydberg 3py的稳定，也涉及占据轨道的不稳定。在7.7 eV的肩部强度在支链烷烃的光谱中急剧增加，特别是对于具有季碳原子的那些，例如2,2-二甲基丁烷。强度的增加是由支链烷烃对称性的降低引起的，这导致禁止向Rydberg 3s的跃迁转变为允许的跃迁。这样，本研究为他们的里德伯格变迁的存在以及这些变迁的相关MO的形状提供了新的见识。

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